Such solar collectors may include an enclosure that encloses the tubes for increasing solar energy absorption and/or for preventing the convection of heat from its structure by the surrounding atmosphere.
U.S. Pat. No. 3,859,980 describes a solar heating unit that has a pair of manifolds and tubes that connect the two manifolds and the entire solar heating unit is enclosed in a clear flexible envelope. The envelope is held spaced from the tubes thereby forming a space both above and below the tubes that provides a sheet of air insulation which prevents the convection of heat from the tubes into the surface on which the unit is mounted and also into the atmosphere above and surrounding the unit.